The long range goal of this project is to use morphological methods such as light and electron microscopy, immunochemical staining, freeze-fracture techniques, and electron dense tracers to study the lamellar structure of myelin and cellular mechanisms of myelin formation and breakdown. Tissues used include neonatal rats, aggregating cultures of mechanically dissociated fetal rat brains, the optic nerves of Xenopus tadpoles, and the myelinated epiretinal fibers of the rabbit eye. Current projects are: 1) use of immunocytochemical techniques to localize myelin constituents during myelin formation and breakdown, 2) an electron microscopic study of aggregating cell cultures during their differentiation and growth in vitro, 3) a study of myelin sheath remodelling in optic nerves of Xenopus tadpoles during metamorphosis, and 4) an investigation of the electron microscopic appearance of interlamellar tight junctions in CNS myelin sheaths.